With the widespread use of portable electric products including video cameras, mobile phones and portable PCs, the importance of secondary batteries primarily used as a power source to power the portable electric products is increasing. Particularly, lithium secondary batteries is increasingly used in a wide range of applications because they have higher energy density per unit weight and allow faster charge than conventional lead accumulators and other secondary batteries such as nickel-cadmium batteries, nickel-hydrogen batteries and nickel-zinc batteries.
Generally, secondary batteries are rechargeable unlike primary batteries that cannot be recharged, and with the development of digital cameras, cellular phones, laptop computers and hybrid vehicles in the high-tech field, many studies have been made on secondary batteries. Secondary batteries include nickel-cadmium batteries, nickel-metal hydride batteries, nickel-hydrogen batteries and lithium secondary batteries. Among them, lithium secondary batteries have the operating voltage of 3.6V or higher and can be used as a power source of portable electronic devices or in high-output hybrid vehicles when connected in series, and there is a fast growing trend in using lithium secondary batteries whose the operating voltage is three times higher and the characteristics of energy density per unit weight are better than nickel-cadmium batteries or nickel-metal hydride batteries.
Lithium secondary batteries can be classified into lithium ion batteries using a liquid electrolyte and lithium ion polymer batteries using a polymer solid electrolyte according to the type of electrolyte. Furthermore, lithium ion polymer batteries can be classified into all-solid-state lithium ion polymer batteries containing no electrolyte solution and lithium ion polymer batteries using a gel-type polymer electrolyte containing an electrolyte solution according to the type of polymer solid electrolyte.
In the case of lithium ion batteries using a liquid electrolyte, a cylindrical or prismatic metal can hermetically sealed by welding is usually used as a container. Can-type secondary batteries using a metal can as a container have a fixed shape, so this disadvantage restricts the design of electric products using such batteries as a power source and it is difficult to reduce the volume. Accordingly, pouch-type secondary batteries fabricated by putting two electrodes, a separator and an electrolyte in a pouch made of a film and sealing were developed and is being used.
FIG. 1 shows an embodiment of a conventional pouch-type secondary battery, in which a pouch 100 is largely divided into a lower sheet 20 and an upper sheet 10 that is situated over it, and an electrode assembly 30 that is embedded in the pouch 100 is formed by stacking and winding a positive electrode, a negative electrode and a separator. After the electrode assembly is received inside, the upper sheet 10 and the lower sheet 20 are sealed along the edges by heat fusion to form a sealing part 23. Furthermore, tabs 37, 38 drawn from each electrode are bonded to electrode leads 39, 40, and a tape 41 may be attached to the electrode leads 39, 40 at an overlapping part with the sealing part 23.
The upper sheet 10 is representatively disclosed for explaining the structure of the pouch having the upper sheet 10 and the lower sheet 20. The upper sheet 10 has a multilayered film structure including an inner layer or a polyolefin-based resin layer that can be heat-welded and serves as a sealant, a metal layer or an aluminum layer that serves as a substrate for maintaining the mechanical strength and a barrier layer against moisture and oxygen, and an outer layer (generally a nylon layer) that acts as a substrate and a protective layer, stacked in a sequential order. For the polyolefin-based resin layer, casted polypropylene (CPP) is commonly used.
Advantages of pouch-type secondary batteries are that there is flexibility in shape and secondary batteries having the same capacity can be realized with smaller volume and mass. However, as opposed to can-type secondary batteries, pouch-type secondary batteries have weak mechanical strength and moisture permeation risks due to using a pouch of a soft material as a container, and have safety problems with explosions of the batteries that may occur due to high temperature and high voltage inside of the batteries caused by abnormal battery operation, for example, internal shorts, overcharge beyond the allowed current and voltage, exposure to high temperature, and impacts caused by falling. Especially, automobile batteries require high capacity, and as the capacity is higher, the safety is poorer.
Furthermore, conventional pouch-type secondary batteries include an electrolyte solution in a pouch, and vapor-phase molecules of the electrolyte solution can penetrate into a sealing part by diffusion in the sealing process. Local defects or micro-cracks by the electrolyte solution molecules penetrated into the sealing part become an electrical path to the outside, causing destruction of insulation resistance of the batteries.
Many studies have been made to improve safety of batteries, but there are still rooms for improvements.